As electronic devices become smaller-sized, higher-performance, and more sophisticated, there is a demand for a printed wiring board capable of providing a high density package. The high density package of the printed wiring board may be achieved by a multilayer wiring board, a thinner insulating layer, an inner via hole instead of a conventional through hole, a small via diameter, a narrower circuit pitch, and the like. A build-up multilayer printed wiring board producing technique is one of techniques for achieving the high density package of the printed wiring board.
Makers produce multilayer printed wiring boards by various build-up methods. Among them, a method, which employs copper foil with an insulating adhesive, is adopted by a number of makers because the material is easy to handle and a conductor layer is already formed and therefore processing steps can be significantly reduced.
The copper foil with an insulating adhesive has a two-layer structure including copper foil and an adhesive layer, which is generally produced by applying a liquid adhesive onto copper foil and drying the adhesive. For example, a general build-up multilayer wiring board using copper foil with an insulating adhesive is produced as follows (see FIG. 1).
Initially, a circuit 7 is formed in advance. Thereafter, copper foil 5 with an insulating adhesive 3 is laminated onto a copper-clad laminate 1 containing glass cloth provided with a through hole by presswork, roll lamination, or the like. Thereafter, a portion of the copper foil in which a via 9 is to be formed are removed by etching using a photosensitive resin, and the adhesive layer is removed by laser drilling, resulting in formation of the via 9. The formed via is made conductive by electroless plating 11. The thus-laminated copper foil is etched to form a circuit 7′.
Again, copper foil with an insulating adhesive is laminated onto the resultant circuit, and thereafter, similar steps to those described above are repeated, thereby producing a multilayer wiring board 20.
However, it is pointed out that as superior properties are required for a build-up multilayer wiring board, several problems arise when using conventional copper foil with an insulating adhesive. Specifically, i) when a narrow pitch circuit pattern is produced by etching, the smaller thickness of a conductor layer to be etched is advantageous in terms of yield, but since the copper foil of conventional copper foil with an insulating adhesive generally has a thickness of 18 μm, or at least 12 μm, there is a limit to the production of a narrow pitch circuit pattern; ii) when a small-diameter via is formed by laser drilling, a smaller aspect ratio, i.e., smaller thickness of the insulating layer, is advantageous, but when a smaller thickness of the insulating layer of conventional copper foil with an insulating adhesive makes it difficult to cause each of the laminated insulating layers to have a uniform thickness, and the insulation performance is reduced, thereby making it difficult to provide electrical reliability; and the like.
In particular, in a multilayer wiring board for which smaller size and lighter weight are required, a glass cloth-containing copper-clad laminate used as a core layer typically has a thickness of about 400 μm and a heavy weight due to the contained glass, and therefore is not a material suitable for reduction of size and weight.
If only copper foil with an insulating adhesive is used without using the glass cloth-containing copper-clad laminate to produce a build-up multilayer wiring board, it is possible to obtain one which has a smaller size and a lighter weight, but the resultant wiring board lacks rigidity, and also lacks package reliability due to a great coefficient of linear expansion derived from the insulating adhesive, and does not endure use.
Japanese Laid-Open Publication No. 8-186376 discloses a method for producing a high density thin multilayer wiring board, in which an organic resin film composite comprising an organic resin film having a thickness of 10 μm to 50 μm and a relative permittivity of 2.2 to 4.0, a wiring pattern having a thickness of 5 μm to 30 μm formed on the front surface thereof, and an adhesive layer having a thickness of 10 μm to 30 μm and a relative permittivity of 2.2 to 4.0 formed on the rear surface thereof, is subjected to thermocompression bonding after lamination so as to provide interlayer adhesion and curing of the adhesive layer. In the method disclosed in Japanese Laid-Open Publication No. 8-186376, the adhesive layer and the organic resin film, which has a coefficient of linear expansion which is different by no more than 8 ppm/° C. from that of the wiring pattern, are used, thereby removing occurrence of warpage and misalignment of the wiring pattern. The adhesive layer is made of a mixture of an epoxy polymer (a monomer or an oligomer) and a polyimide or a polyamide. The adhesive layer is formed by drying at 90° C. for 30 minutes before the lamination, and is cured during thermocompression bonding.
Japanese Laid-Open Publication No. 9-139558 discloses a bare-chip packaging substrate in which a conductive wire is formed directly on a surface of an organic polymer film having a coefficient of linear expansion of −5×10−6/K to 5×10−6/K and a modulus of elasticity (Young's modulus) of at least 7 GPa. The organic polymer film is laminated via an adhesive film having a modulus of elasticity (Young's modulus) of no more than 7 GPa. By reducing thermal expansion of the organic polymer film, the packaging substrate suppresses occurrence of cracking or peeling.
Japanese Laid-Open Publication No. 9-55583 discloses a multilayer printed wiring board comprising an insulating film material which exhibits adhesiveness by heating, where the adhesion temperature, at which the adhesiveness appears, is lower than the adhesive temperature of a conductive bonding material, thereby preventing wire breakage.
Japanese Laid-Open Publication No. 11-148053 discloses a heat resistant plastic film multilayer structure in which an adhesive layer is formed on a side of a heat resistant plastic film, where the resin flow of the adhesive layer is 3.0 to 5.0% so that the roughness of an inner layer circuit does not appear on a surface.